1. Field of the Invention
This invention relates to an optical displacement detection apparatus for measuring displacement information, such as the amount of displacement, the moving speed, or the like, of a moving object, utilizing the fact that when a light beam projected onto the moving object is diffracted and scattered, diffracted and scattered light beams are subjected to phase modulation in accordance with the displacement or the moving speed of the object.
2. Description of the Related Art
As conventional measuring apparatuses for obtaining a physical quantity, such as the displacement, the moving speed, or the like, of a moving object with high accuracy by projecting a light beam onto the object, there are, for example, optical encoders, laser Doppler velocity meters, and laser interferometers. In a field other than that of measuring apparatuses, there are optical pickups for reading digital information recorded in an object, such as a compact disc, as fine projections and recesses by projecting a light beam onto the projections and recesses.
In each of these apparatuses, a light beam emitted from a light-emitting device in accordance with an electrical signal is projected onto an object, the light beam reflected by the object is guided to and sensed by a photosensor via optical components, and the photosensor converts the sensed light beam into an electrical signal representing recorded information.
FIGS. 1 and 2 illustrate a conventional optical encoder disclosed, for example, in Japanese Utility Model Laid-open Application (Kokai) No. 1-180615 (1989). In FIGS. 1 and 2, a light beam emitted from a light-emitting device 42 (denoted by reference numeral 18 in FIG. 2) is converted into a linear light-source array by a slit array 14 (the pitch of which is P.sub.2), and is projected onto a grid 12 (the pitch of which is P.sub.1) on a scale 40. The grid of the scale 40 is projected onto an index grid 16 (the pitch of which is P.sub.3) by the light beam reflected by the grid on the scale 40. The amount of light incident upon a photosensor 48 after passing through the index grid 16 is modulated by the geometrical overlap of the two grids.
Although not disclosed in this publication, the size of the encoder can be reduced by encapsulating the light-emitting device 42 and the photosensor 48 on chips within a receptacle. However, the resolution and the accuracy of an encoder having the above-described detection principle are a few micrometers at most. Such a value is insufficient for realizing high accuracy and high resolution.
FIGS. 3(a) and 3(b) illustrate other conventional optical encoders disclosed in Japanese Patent Laid-open Application (Kokai) No. 62-121314 (1987). Each of these encoders is an example of very effective improvements for reducing the size of the basic optical system of an encoder using three diffraction gratings (disclosed in British Patent Laid-open Application No. 1474049).
A light beam emitted from a light-emitting device L is made into a parallel light beam by a lens 50, and is projected onto and diffracted by a grating having grating constant GK and having scanning fields AF and AF provided on an index scale A, whereby light beams are generated in three directions. Each of these light beams is diffracted by a grating having grating constant GK provided on a scale B, and is returned to the grating GK on the index scale A by being subjected to phase modulation caused by relative movement. Three interference light beams emanate in different directions as a result of diffraction by the grating GK on the index scale A, are condensed onto different positions by the lens 50, and are sensed by photosenors C provided at respective positions.
In the above-described optical encoder, the separation and synthesis of diffracted light beams are performed by the same grating on the index scale A and lens 50. Accordingly, if the pitch of the scale is reduced to about a few micrometers, the optical paths tend to be separated. If it is intended to cover the separated optical paths by a single lens 50, the diameter of the lens 50 must be increased, thereby causing difficulty in reducing the size of the encoder. Furthermore, since signals having phase differences are provided by the shape of the cross section (the step and the ratio of projections to recesses) of the index grating on the index scale A, it is extremely difficult to process the grating if the pitch of the grating is fine. That is, the configuration of the above-described encoder is disadvantageous for realizing the compatibility of a small size o the order of millimeters, and high precision and high resolution on the order of 0.1 .mu.m.
FIG. 4 is a cross-sectional view illustrating the schematic configuration of a linear encoder having a small size and high precision described in U.S. Pat. No. 5,283,434 filed by the assignee of the present application. In FIG. 4, a package, in which a semiconductor laser 1 is sealed, is accommodated in a holder 5. A glass-epoxy substrate 3, in which a threaded hole 4, capable of transmitting light emitted from the semiconductor laser 1, is formed, and on which photosensors 2B and 2C are provided, is laminated at a predetermined position on the holder 5. An optical-unit holder 6 is laminated on the glass-epoxy substrate 3, and an optical unit 7 having a planoconvex lens 8 on one surface thereof, and diffraction gratings 9A, 9B and 9C formed using a replica technique on another surface thereof, is laminated at a predetermined position on the optical-unit holder 6. A scale 10 is provided facing a surface, having the diffraction gratings formed thereon, of the optical unit 7.
In the optical displacement sensor head having the above-described configuration, a divergent laser beam emitted from the semiconductor laser 1 passes through the threaded hole 4, and is incident upon the planoconvex lens 8 of the optical unit 4 to be made into a parallel light beam. The parallel light beam is separated into light beams of 0-order, .+-. first-order, . . . by the diffraction gratings 9A, 9B and 9C formed on the other surface of the optical unit 7. The respective separated light beams are incident upon the scale 10, and are diffracted by a diffraction grating 10A provided on the scale 10 and are reflected by a reflecting film formed on the scale 10, and return to the diffraction gratings 9A, 9B and 9C on the optical unit 7.
At that time, by optimizing the diffraction angles of respective light beams, the gap between the optical displacement sensor head and the scale, and the thickness of the scale, the 0-order light beam and the .+-. first-order light beams return to the same region on the optical unit 7, and are synthesized by the diffraction gratings 9A, 9B and 9C on the optical unit 7. At that time, signal light beams interfere with each other with phase differences produced in accordance with the amount of movement of the scale 10, whereby the amounts of respective light beams change. A signal light beam as the result of the synthesis is incident upon the photosensing surfaces of the photosensors 2B and 2C.
By removing a useless space and arranging optimum optical components in respective optical paths according to the above-described configuration, it is possible to obtain a small high-performance optical displacement sensor head.